The present invention relates to a method for separating p-xylene by separating and recovering only p-xylene from a p-xylene-containing raw material mixture. More particularly, the present invention relates to a method for separating p-xylene using a zeolite membrane as a separating membrane.
Xylene (dimethylbenzene) has three isomers (o-xylene, m-xylene and p-xylene) depending upon the positions of two methyl groups. Among the three isomers, p-xylene (which is a 1,4-substitution product of benzene) is particularly important because it, when oxidized, becomes terephthalic acid which is industrially useful as a starting material for polyesters such as PET, PBT and the like.
p-Xylene, together with o-xylene, m-xylene and ethylbenzene, is present in, for example, the C-8 fraction of coal tar. Since these compounds have the same molecular weight and are close in boiling point, it is impossible to separate p-xylene from their mixture by fractional distillation alone. Therefore, methods are under study which comprise separating and recovering only p-xylene from a raw material mixture using, as a separating membrane, a membrane composed of zeolite known as a molecular sieve.
For example, JP-B-5-63410 discloses a method for separating only p-xylene selectively from a m-xylene/p-xylene mixture using a ZSM-5 (MFI) type zeolite membrane formed, in a thin film, on a porous glass (a substrate). This method is hereinafter referred to as xe2x80x9cthe first method.xe2x80x9d
Also, WO 93/17781 discloses a method for separating only p-xylene selectively from an equal volume mixture of m-xylene, p-xylene and triisopropylbenzene using a ZSM-5 (MFI) type zeolite membrane under the conditions of room temperature and a total pressure (at raw material side) of 1,720 kPa (p-xylene partial pressure=573 kPa). This method is hereinafter referred to as xe2x80x9cthe second method.xe2x80x9d
Further, WO 94/25151 discloses a method for separating only p-xylene selectively from a o-xylene/p-xylene mixture using a MFI type zeolite membrane under the conditions of 100 to 200xc2x0 C. and a p-xylene partial pressure (at raw material side) of 0.31 kPa. This method is hereinafter referred to as xe2x80x9cthe third method.xe2x80x9d
The above separation methods are useful in that they can separate p-xylene selectively; however, they have a problem in that the amount of p-xylene permeating through the zeolite membrane is small. Hereinafter, the amount is referred to as xe2x80x9cp-xylene permeation amount.xe2x80x9d
In order to make the separation of p-xylene using zeolite industrially applicable, it is necessary to separate and recover a large amount of p-xylene efficiently (so as to match the cost and equipment used) under the high-temperature and high-pressure conditions of 200xc2x0 C. or higher and 25 kPa or higher (a p-xylene partial pressure at the raw material side); that is, a large p-xylene permeation amount is required. However, none of the above methods are fully satisfactory in providing a large p-xylene permeation amount.
The second method is a method for separating p-xylene at a low temperature (room temperature); the third method is a method for separating p-xylene at a low pressure (the p-xylene partial pressure at raw material side is 0.31 kPa); and the first method makes no mention at all about separation conditions or p-xylene permeation amount and therefore shows no study based on p-xylene permeation amount.
Thus, as to a method for separation of p-xylene using a zeolite membrane, there is no industrial application yet.
In view of the above-mentioned problems of the prior art, the present invention is intended to provide a method for separating p-xylene, which can secure a sufficient p-xylene permeation amount and has industrial applicability.
The present inventors made an in-depth study on p-xylene adsorption on zeolite under a high temperature condition, in order to obtain an increased p-xylene permeation amount. As a result, the present inventors surprisingly found out a fact that at a low p-xylene partial pressure region, the p-xylene adsorption increases sharply with an increase in p-xylene partial pressure, but the increase becomes strikingly small when the p-xylene partial pressure is at a certain level or larger. The present invention has been completed based on the above finding.
According to the present invention, there is provided a method for separating p-xylene, which comprises separating and recovering only p-xylene from a p-xylene-containing raw material mixture under high-temperature and high-pressure conditions using a zeolite membrane as a separating membrane, in which method the p-xylene partial pressure at the raw material side of the separating membrane is kept at a sufficiently high pressure and the p-xylene partial pressure at the recovery side of the separating membrane is controlled at a pressure which is not higher than the inflection point of a p-xylene adsorption curve.
In the separation method of the present invention, the p-xylene partial pressure at the raw material side of the separating membrane is controlled preferably at a pressure higher than the inflection point of a p-xylene adsorption curve; it is preferred that the separation is conducted at 200xc2x0 C. or higher, the p-xylene partial pressure at the raw materials side of the separating membrane is kept at 100 kPa or higher, and the p-xylene partial pressure at the recovery side of the separating membrane is controlled at 20 kPa or lower; and the separating membrane is preferably a zeolite membrane composed of a MFI type zeolite.
In the separation method of the present invention, the p-xylene partial pressure at the raw material side of the separating membrane is kept at a sufficiently high pressure and the p-xylene partial pressure at the recovery side of the separating membrane is controlled at a pressure which is not higher than the inflection point of p-xylene adsorption curve. Therefore, the present separation method can secure a sufficient p-xylene permeation amount and has industrial applicability.